Aluminum alloy material and production method therefor

ABSTRACT

An aluminum alloy material as one aspect of the present disclosure has a chemical composition including: Zn: more than 6.5% (mass %, same applies hereafter) and 8.5% or less; Mg: 0.5% or more and 1.5% or less; Cu: 0.10% or less; Fe: 0.30% or less; Si: 0.30% or less; Mn: less than 0.05%; Cr: less than 0.05%; Zr: 0.05% or more and 0.10% or less; and Ti: 0.001% or more and 0.05% or less, a balance including Al and inevitable impurities. In the aluminum alloy material, a mass ratio of Zn to Mg (Zn/Mg) is 5 or more and 16 or less, and a metallographic structure includes an equigranular recrystallized structure.

CROSS-REFERENCE TO RELATED APPLICATION

This international application claims the benefit of Japanese PatentApplication No. 2015-227926 filed on Nov. 20, 2015 with the Japan PatentOffice, and the entire disclosure of Japanese Patent Application No.2015-227926 is incorporated herein.

TECHNICAL FIELD

The present disclosure relates to an aluminum alloy material and aproduction method therefor.

BACKGROUND ART

Conventional 7000-series aluminum alloys with Zn and Mg added to Al havebeen known as aluminum alloys exhibiting a high strength. Such7000-series aluminum alloys exhibit a high strength due to ageprecipitation of Al—Mg—Zn-based fine precipitates. 7000-series aluminumalloys to which Cu has been added in addition to Zn and Mg exhibit thehighest strength among aluminum alloys.

7000-series aluminum alloys are produced by, for example, hot extrusionor other process, and are used in applications requiring a highstrength, including transportation equipment, such as aircraft andvehicles, and machine parts, as well as sporting goods and so on.Properties that 7000-series aluminum alloys are required to have whenused in such applications include impact absorbability (toughness),resistance to stress corrosion cracking (hereinafter referred to asresistance to SCC, which is an abbreviation of Stress CorrosionCracking), and so on, in addition to strength. Proposed as an example of7000-series aluminum alloys is, for example, an aluminum alloy extrudedmaterial disclosed in Patent Document 1.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 2007-119904

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In 7000-series aluminum alloys, when an amount of Zn and Mg added toachieve a high strength is increased, a strength improving effect isobtained, whereas a problem of decrease in workability, such asextrusion processability, arises.

Further, in the above-described applications, good appearance propertiesare required in addition to the above-described various properties;thus, surface quality such as surface texture and visual appearance isregarded as important. In general 7000-series aluminum alloys, when asurface treatment such as anodization is performed for the purpose ofpreventing surface scratches, compounds precipitated on a grain boundaryare preferentially etched at pretreatment, whereby streak patterns orthe like are generated on the surface-treated surface, resulting in aproblem in surface quality. Especially in a case where themetallographic structure is made to be fibrous in order to obtain ahigher strength, such streak patterns are conspicuous because thecompounds precipitated on the grain boundary are arranged along thefibrous metallographic structure. As a result, it is difficult to obtaina good surface quality.

Means for solving the above-described problems in surface quality, suchas generation of the streak patterns, include to make the metallographicstructure to be a recrystallized structure, which is not fibrous butequigranular. With such a recrystallized structure, a situation can beinhibited in which the compounds precipitated on the grain boundary arearranged linearly, whereby generation of streak patterns can be reduced.However, it is known that, in the case where a 7000-series aluminumalloy has the recrystallized structure, its strength is lowered and itstoughness and resistance to SCC are also decreased in some cases, ascompared with the case of having the fibrous structure. In addition,with the recrystallized structure, scale-like patterns are conspicuousalthough generation of the streak patterns can be reduced. In this way,conventional 7000-series aluminum alloys have been difficult to use inthe applications requiring properties such as resistance to SCC andsurface quality as well, in addition to a high strength and a hightoughness.

In one aspect of the present disclosure, it is desirable to provide ahigh-strength aluminum alloy material that is excellent in surfacequality, toughness, and resistance to SCC; and a production methodtherefor.

Means for Solving the Problems

An aluminum alloy material as one aspect of the present disclosure has achemical composition comprising: Zn: more than 6.5% (mass %, sameapplies hereafter) and 8.5% or less; Mg: 0.5% or more and 1.5% or less;Cu: 0.10% or less; Fe: 0.30% or less; Si: 0.30% or less; Mn: less than0.05%; Cr: less than 0.05%; Zr: 0.05% or more and 0.10% or less; and Ti:0.001% or more and 0.05% or less, a balance comprising Al and inevitableimpurities. In the aluminum alloy material, a mass ratio of Zn to Mg(Zn/Mg) is 5 or more and 16 or less, and a metallographic structurecomprises an equigranular recrystallized structure.

The above-described aluminum alloy material has the above-specifiedchemical composition, and its metallographic structure comprises theequigranular recrystallized structure. This makes it possible to inhibitpoor surface quality after surface treatment such as anodization, ascompared with a case in which its metallographic structure is a fibrousstructure. In particular, regulation of the upper limit of the Mgcontent makes it possible to inhibit precipitation of the compounds onthe grain boundary while ensuring a high strength, thereby inhibitinggeneration of scale-like patterns on the surface caused by therecrystallized structure after surface treatment such as anodization.Moreover, regulation of the upper limit of the Cu content makes itpossible to inhibit the surface from becoming yellowish in color tone bysurface treatment. As a result, a good surface quality can be obtained.Furthermore, by setting the mass ratio of Zn to Mg (Zn/Mg) to theabove-specified range, toughness and resistance to SCC can be improvedwhile ensuring a high strength.

A production method for an aluminum alloy material as another aspect ofthe present disclosure is a method for producing an aluminum alloymaterial, a metallographic structure of which comprises an equigranularrecrystallized structure. The method comprises: preparing an ingothaving a chemical composition comprising: Zn: more than 6.5% (mass %,same applies hereafter) and 8.5% or less; Mg: 0.5% or more and 1.5% orless; Cu: 0.10% or less; Fe: 0.30% or less; Si: 0.30% or less; Mn: lessthan 0.05%; Cr: less than 0.05%; Zr: 0.05% or more and 0.10% or less;and Ti: 0.001% or more and 0.05% or less, a balance comprising Al andinevitable impurities, wherein a mass ratio of Zn to Mg (Zn/Mg) is 5 ormore and 16 or less; and performing a homogenizing treatment in whichthe ingot is heated at a temperature higher than 540° C. and 580° C. orlower for 1 hour or longer and 24 hours or shorter.

In the above-described production method for the aluminum alloymaterial, the ingot having the above-specified chemical component andhaving the mass ratio of Zn to Mg (Zn/Mg) set to the above-specifiedrange is prepared in the production process. Then, the ingot issubjected to the homogenizing treatment under the above-specifiedconditions. In particular, by setting the heating temperature in thehomogenizing treatment to a high temperature, which is higher than 540°C. and 580° C. or lower, it becomes possible to easily obtain theabove-described aluminum alloy material, that is, a high-strengthaluminum alloy material, a metallographic structure of which comprisesan equigranular recrystallized structure and which is excellent insurface quality, toughness, and resistance to SCC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing a bending test method.

FIG. 2 is an explanatory diagram showing a method for observingmetallographic structures.

EXPLANATION OF REFERENCE NUMERALS

10 . . . test piece, 20 . . . specimen

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present disclosure will be described below. It isneedless to say that the present disclosure is not limited to thebelow-described embodiments, and that the present disclosure can bepracticed in various forms within the scope not departing from the gistof the present disclosure.

Detailed explanation will be given of a composition of respectivecomponents of aluminum alloy materials in the embodiments of the presentdisclosure.

Zn:

Zn coexists with Mg to precipitate a η′ phase, and provides an effect ofimproving strength. The range of Zn content is more than 6.5% and 8.5%or less. If the Zn content is 6.5% or less, a precipitation amount ofthe η′ phase is reduced, thus decreasing the strength improving effect.In contrast, if the Zn content is more than 8.5%, hot workability isreduced to thereby decrease productivity. A preferred range of the Zncontent is 7.0% or more and 8.0% or less.

Mg:

Mg coexists with Zn to precipitate a η′ phase, and provides the effectof improving strength. The range of Mg content is 0.5% or more and 1.5%or less. In particular, by regulating the upper limit of the Mg contentto 1.5% or less, it is possible to inhibit precipitation of compounds ona grain boundary (a crystal grain boundary, a sub-grain boundary, or thelike), while obtaining the strength improving effect. This makes itpossible to reduce, at the time of surface treatment such asanodization, an amount of the compounds that have precipitated on thegrain boundary to be etched at pretreatment, to thereby inhibitgeneration of scale-like patterns on the surface-treated surface.

If the Mg content is less than 0.5%, a precipitation amount of the η′phase is reduced, thus decreasing the strength improving effect. Incontrast, if the Mg content is more than 1.5%, coarse compounds arelikely to be generated on the grain boundary, thus increasing an amountof the compounds to be etched at pretreatment of surface treatment suchas anodization. Therefore, scale-like patterns are generated on thesurface-treated surface, resulting in poor surface quality. To obtain agood surface quality and a higher strength, the Mg content is preferably1.0% or more and 1.3% or less.

Cu:

Cu may get mixed in when a recycled material is used as a raw materialfor an aluminum alloy material. In a 7000-series aluminum alloy,inclusion of Cu contributes to improvement in strength, whereas changein color tone or the like occurs, such as yellowing of the color tone ofthe surface caused by surface treatment such as anodization. Such changein color tone may cause poor surface quality. Thus, when an emphasis isparticularly placed on the color tone of the surface-treated surface,the upper limit of Cu content needs to be regulated. Regulation of theupper limit of the Cu content to 0.10% or less makes it possible toreduce the above-described poor surface quality. The Cu content ispreferably 0.08% or less.

Fe, Si, Mn, and Cr:

Fe and Si may get mixed in as impurities of aluminum metal. Mn and Crmay get mixed in when a recycled material is used as a raw material foran aluminum alloy material. Of the above-described four components, Fe,Si, and Mn have an effect of inhibiting recrystallization by formingAl—Mn-based, Al—Mn—Fe-based, and/or Al—Mn—Fe—Si-based intermetalliccompounds in combination with Al. Cr has an effect of inhibitingrecrystallization by forming Al—Cr-based intermetallic compounds incombination with Al. Thus, inclusion of the above-described fourcomponents results in inhibiting formation of a recrystallizedstructure, and instead results in formation of a fibrous structure.

That is, excessive inclusion of the above-described four componentsresults in formation of the fibrous structure and, in combination withthe size and distribution of the compounds, streak patterns aregenerated on the surface subjected to surface treatment such asanodization, leading to poor surface quality. Thus, by regulating Fecontent to 0.30% or less, Si content to 0.30% or less, Mn content toless than 0.05%, and Cr content to less than 0.05%, formation of thefibrous structure is inhibited, and the above-described poor surfacequality, specifically generation of the streak patterns, can thereby beinhibited.

Zr:

Zr is added to obtain a fine and uniform recrystallized structure. Therange of Zr content is 0.05% or more and 0.10% or less. Zr forms fineAl—Zr-based compounds in combination with Al. In the process ofproducing the aluminum alloy material, the crystal structure of theAl—Zr-based compounds changes depending on the temperature at which theingot is subjected to homogenizing treatment. If the temperature in thehomogenizing treatment is 540° C. or lower, a metastable phase is formedwhich has an L1₂ structure commensurate with the matrix, thus inhibitingrecrystallization in the structure subjected to hot working and readilyleading to formation of a fibrous structure. In contrast, if thehomogenizing treatment is performed at a temperature higher than 540° C.and 580° C. or lower, the Al—Zr-based compounds change into anequilibrium phase having a D0₂₃ structure. This results in formation ofan equigranular recrystallized structure, not a fibrous structure, afterhot working, and also inhibits recrystallized grains from coarsening byblocking movement of the crystal grain boundary.

If the Zr content is less than 0.05%, the effect of inhibiting therecrystallized grains from coarsening is less likely to be obtained,resulting in formation of a nonuniform metallographic structure in whichthe recrystallized grains have partially coarsened. This causes aproblem that mottled patterns are visually confirmed on the surfacesubjected to surface treatment such as anodization, or other problem,and results in poor surface quality. On the other hand, if the Zrcontent is more than 0.10%, the Al—Zr-based compounds are distributedmore densely; thus, recrystallization is inhibited to form a fibrousstructure. This causes generation of streak patterns on thesurface-treated surface, and results in poor surface quality.

Ti:

Ti is added to seek micronization of crystal grains in the ingot. Therange of Ti content is 0.001% or more and 0.05% or less. If the Ticontent is less than 0.001%, an effect of micronizing the crystal grainsis reduced. Thus, mottled patterns are likely to be generated on thesurface subjected to surface treatment such as anodization, resulting inpoor surface quality. On the other hand, if the Ti content is more than0.05%, a point defect is likely to occur on the surface-treated surfacedue to Al—Ti-based intermetallic compounds formed in combination with Alor other cause, resulting in poor surface quality.

Other Elements:

Contained other than the above-listed elements may be basically Al andinevitable impurities. Elements to be generally added to the aluminumalloy other than the above-listed elements are allowed to be present asinevitable impurities, within a range not greatly affecting theproperties of the aluminum alloy.

In the above-described aluminum alloy material, the mass ratio of Zn toMg (Zn/Mg) is 5 or more and 16 or less. As described above, 7000-seriesaluminum alloys can generally obtain higher strength by addition of Znand Mg. However, addition of a large amount of Zn reduces hotworkability, and addition of a large amount of Mg facilitates generationof coarse compounds to thereby reduce surface treatmentability andtoughness. Further, general 7000-series alloys are known as having adecreased resistance to SCC when the metallographic structure thereof isa recrystallized structure. In the present disclosure, upper limits ofthe Zn content and the Mg content are regulated and, further, the massratio (Zn/Mg) is set to be within the above-specified range. As aresult, the following properties can be obtained.

Specifically, by regulating the upper limits of the Zn content and theMg content, the absolute value of the generation amount of MgZn₂compounds is made smaller. Further, by setting the mass ratio (Zn/Mg) to16 or less, that is, by decreasing the Mg content relatively and also byregulating the mass ratio (Zn/Mg) to 16 or less, the MgZn₂ compounds areinhibited from growing coarse. As a result, fine compounds are obtainedand toughness can be improved.

The resistance to SCC will be discussed below. In general 7000-seriesaluminum alloys, an electric potential of the matrix in the vicinity ofthe grain boundary is nobler than that of the MgZn₂ compoundsprecipitated on the grain boundary. Such an electric potentialdifference causes a local anodic dissolution under a stress corrosionenvironment, thus generating a crack in the vicinity of the grainboundary. This is considered to cause stress concentration and, thus,generation and progress of cracking. In the present disclosure, the massratio (Zn/Mg) is set to 5 or more, that is, an amount of Zn that issolid-solved in the matrix is made to be relatively large and also themass ratio (Zn/Mg) is regulated to 5 or more. This makes it possible toalleviate the electric potential difference from the MgZn₂ compoundspresent on the grain boundary, thus improving the resistance to SCC evenin the recrystallized structure.

As described above, in the present disclosure, a high-strength aluminumalloy material, which has a good surface quality and is excellent intoughness and resistance to SCC can be obtained by regulating the upperlimits of the Zn content and the Mg content and also by setting the massratio (Zn/Mg) to 5 or more and 16 or less.

In the above-described ranges of the Zn content and the Mg content, ifthe mass ratio (Zn/Mg) is less than 5, the effect of reducing andmicronizing the compounds composed of Zn and Mg is decreased, and theeffect of improving toughness cannot be sufficiently obtained. On theother hand, if the mass ratio (Zn/Mg) is more than 16, the Zn contentbecomes larger to thereby cause anodic dissolution in the vicinity ofthe grain boundary more likely, resulting in decrease in resistance toSCC. A preferable range of the mass ratio (Zn/Mg) is 7 or more and 16 orless.

The metallographic structure of the above-described aluminum alloymaterial comprises an equigranular recrystallized structure. Therecrystallized structure means a metallographic structure comprisingequigranular recrystallized grains. The metallographic structure can beconfirmed by, for example, observing a surface or a cross-section of thealuminum alloy material with a polarizing microscope.

In the above-described aluminum alloy material, it is preferable thatthe recrystallized structure be such that: an average grain diameter ofthe crystal grains in a cross-section parallel to a direction orthogonalto a working direction of the aluminum alloy material (e.g., a directionof extrusion in the case of an extruded material) is 500 μm or less; andalso such that a difference between the maximum value and the minimumvalue of the grain diameters of the crystal grains is less than 300 μm.In this case, the grain diameters of the crystal grains in therecrystallized structure are more uniform, and a good surface quality isthereby obtained. “Working” as in the “working direction” meansextruding, rolling, or other processing. The “cross-section parallel toa direction orthogonal to a working direction” means, for example, across-section parallel to a width direction (a cross-section orthogonalto a thickness direction) when the working direction is assumed to be alength direction.

If the average grain diameter of the crystal grains in therecrystallized structure is more than 500 μm, the crystal grains areexcessively coarse, resulting in a risk that mottled patterns caused bythe coarse crystal grains may be generated on the surface subjected tosurface treatment such as anodization. If the difference between themaximum value and the minimum value of the grain diameters of thecrystal grains is 300 μm or more, the metallographic structure isnonuniform, resulting in a risk that a light reflection state may benonuniform on the surface subjected to surface treatment.

The yield strength, as defined in JIS Z2241 (ISO 6892-1), of theabove-described aluminum alloy material is preferably 300 MPa or more,and more preferably 350 MPa or more. This makes it possible torelatively easily obtain strength properties applicable to a lesser wallthickness for weight reduction.

Next, in a production method for the above-described aluminum alloymaterial, an ingot is prepared which comprises the above-describedchemical components and in which the mass ratio of Zn to Mg (Zn/Mg) is 5or more and 16 or less, and then a homogenizing treatment is performedin which the ingot is heated at a temperature of higher than 540° C. and580° C. or lower for 1 hour or longer and 24 hours or shorter.

If the heating temperature in the above-described homogenizing treatmentis 540° C. or lower, the Al—Zr-based compounds present in the ingot forma metastable phase having an L1₂ structure commensurate with the matrix,thus inhibiting recrystallization in the structure subjected to hotworking and readily leading to formation of a fibrous structure. Thiscauses generation of streak patterns on the surface subjected to surfacetreatment such as anodization, and results in poor surface quality.Further, a segregated layer in the ingot is not homogenized, and thestructure subjected to hot working becomes a nonuniform recrystallizedstructure. As a result, a final surface quality becomes similarly poor.On the other hand, if the heating temperature in the above-describedhomogenizing treatment is higher than 580° C., the ingot may be meltlocally, resulting in difficulty in practical production.

Accordingly, the heating temperature in the above-described homogenizingtreatment is set to be higher than 540° C. and 580° C. or lower, wherebythe Al—Zr-based compounds present in the ingot change to an equilibriumphase having a D0₂₃ structure. This results in formation of anequigranular recrystallized structure, not a fibrous structure, afterhot working, and also inhibits the recrystallized grains from coarseningby blocking movement of the crystal grain boundary.

If the heating time for the above-described homogenizing treatment isshorter than 1 hour, the segregated layer in the ingot is nothomogenized, and the structure subjected to hot working becomes anonuniform recrystallized structure. As a result, a final surfacequality becomes poor similarly to the above. On the other hand, if theheating time for the above-described homogenizing treatment exceeds 24hours, the segregated layer in the ingot is sufficiently homogenized;thus, no further effect can be expected. Accordingly, the heating timefor the above-described homogenizing treatment is set to 1 hour orlonger and 24 hours or shorter.

The above-described aluminum alloy material includes, for example, anextruded material, a plate material, and so on made of aluminum alloy.The present disclosure can be applied to various aluminum alloymaterials and production methods therefor.

EXAMPLES Example 1

Examples of the aluminum alloy material of the present disclosure willbe described through comparison with comparative examples, withreference to Table 1 and Table 2. The below-described examples show oneembodiment of the present disclosure, and the present disclosure is notlimited to these.

As shown in Table 1 and Table 2, a plurality of specimens of thealuminum alloy material (examples: Specimen 1 to Specimen 23,comparative examples: Specimen 24 to Specimen 38) containing differentchemical components were prepared under the same production conditions,and various evaluations were conducted on each specimen. A preparationmethod and various evaluation methods for the specimens will bedescribed below.

<Method for Preparing Specimen>

A cylindrical ingot (billet) having a diameter of 90 mm containingchemical components shown in Table 1 is forged by semicontinuouscasting. Then, a homogenizing treatment is performed in which the ingotis heated at 560° C. for 12 hours. The heating temperature in thehomogenizing treatment may be higher than 540° C. and 580° C. or lower.Subsequently, the ingot is subjected to hot extrusion with thetemperature of the ingot maintained at 520° C. In this way, an extrudedmaterial having a width of 150 mm and a thickness of 10 mm is obtained.

Next, a quenching treatment is performed in which the extruded materialsubjected to hot extrusion is cooled to 100° C. at a cooling rate of1500° C./min. Then, after the quenched extruded material is cooled toroom temperature, an artificial aging treatment is performed in whichthe extruded material is heated at 140° C. for 12 hours. In this way, aspecimen of the aluminum alloy material (extruded material) is obtained.

<Method for Evaluating Mechanical Properties>

A test piece is prepared from the specimen by a method based on JISZ2241 (ISO 6892-1), and a tensile strength, a yield strength, and anelongation of the test piece are measured. The test piece having a yieldstrength of 300 MPa or more is determined to be acceptable. Thecriterion for determining the yield strength is just an example.

As for a bending test, as shown in FIG. 1, a test piece 10 having athickness of 10 mm, a width of 10 mm, and a length of 120 mm is preparedfrom a width-direction central portion of the specimen, and an amount Δof bending deformation of the test piece 10 is measured by a three-pointbending test. Specifically, a jig comprising a base 11 and twosupporting portions 12 is prepared, and the test piece 10 is left atrest on the two supporting portions 12. At this time, the two supportingportions 12 each support the test piece 10 at a position 10 mm from thecorresponding end of the test piece 10, so that a distance betweensupporting points becomes 100 mm. Then, a downward load in a directionorthogonal to the width direction of the specimen is applied to thespecimen by an indenter 13, the dimension of which at a leading endsurface is 10 mm×10 mm. Here, if the amount Δ of bending deformationafter application of the load of 4000 kgf for 10 seconds is more than 4mm, the test piece 10 is determined to be unacceptable “X”; if more than2 mm and 4 mm or less, the test piece 10 is determined to be acceptable“◯”; and if 2 mm or less, the test piece 10 is determined to bedesirable “⊚”.

<Method for Evaluating Toughness>

A Charpy impact test is performed by a method based on JIS Z2242.Specifically, a test piece having a thickness of 7.5 mm, a width of 10mm, and a length of 55 mm is prepared. A longitudinal direction of thetest piece is parallel to a direction of extrusion, and the test piecehas a U-shaped notch having a depth of 2 mm, formed so as to beorthogonal to the direction of extrusion. The Charpy impact test isperformed on the test piece, and an impact value is measured. If theimpact value is 15 J/cm² or more, the test piece is determined to beacceptable, and if less than 15 J/cm², the test piece is determined tobe unacceptable. The criteria for determining the impact value is justan example.

<Method for Evaluating Resistance to SCC>

An SCC test is performed by a method based on JIS Z8711. Specifically, atest piece having a C-ring shape (outside diameter: 19 mm, insidediameter: 16 mm, thickness: 8 mm) is prepared. Then, a stress of 90% ofthe yield strength is applied to the test piece such that a direction ofapplication of a tensile stress at a stress-concentrated partcorresponds to a direction of extrusion of the test piece. In such astate and under a temperature environment of 25° C., the test piece isimmersed in salt water with the concentration of 3.5% for 10 minutes andthen dried for 50 minutes. Such steps as one cycle are repeatedlyperformed. Thirty days later, whether a cracking is generated in thetest piece is visually confirmed. If no cracking is generated, the testpiece is determined to be acceptable, and if a cracking is generated,the test piece is determined to be unacceptable.

<Method for Observing Metallographic Structure>

A texture observation of the specimen is performed at a cross-sectionparallel to a width direction when the working direction (the directionof extrusion here) is assumed to be a length direction. In particular, aportion in the vicinity of a width-direction center of the cross-sectionis observed. As shown in FIG. 2, an extruded material 20 as the specimenis cut, and three cross-sections in total, that is, a cross-section at athickness-direction central position of the extruded material 20 andcross-sections at ¼ positions from the top and the bottom in thethickness directions of the extruded material 20, are electrolyticallypolished. Then, a microscopic image (e.g., a photograph shown in a lowerpart of FIG. 2) of each cross-section at 50 to 100-fold magnification isobtained using a polarizing microscope. Subsequently, whether themetallographic structure is an equigranular recrystallized structure isconfirmed from the obtained microscopic image. If the metallographicstructure is fibrous, the specimen is determined to be acceptable. Ifthe metallographic structure is nonuniform, the specimen is determinedto be unacceptable. As shown in FIG. 2, a direction of observation isthe thickness direction of the specimen.

Furthermore, as for the specimen whose metallographic structure is anequigranular recrystallized structure, the obtained microscopic imagethereof is subjected to image analysis. Equivalent circle diameters ofthe crystal grains on the respective cross-sections are found, and anaverage grain diameter of the crystal grains on each cross-section iscalculated. In addition, the greatest diameters and the smallestdiameters of the crystal grains on the respective cross-sections arefound, and the greatest one of the greatest diameters and the smallestone of the smallest diameters are respectively referred to as a maximumvalue and a minimum value. Then, a difference between the maximum valueand the minimum value of the grain diameters of the crystal grains (agrain diameter difference) is calculated. If the average grain diameterof the crystal grains on each cross-section is 500 μm or less and thedifference between the maximum value and the minimum value of the graindiameters of the crystal grains on all the cross-sections observed (thegrain diameter difference) is less than 300 μm, the specimen isdetermined to be desirable.

<Method for Evaluating Surface Quality>

After a surface of the specimen is mechanically polished (buffed), thespecimen is etched with an aqueous sodium hydroxide and is furtherdesmutted. Then, the desmutted specimen is chemically polished by aphosphoric acid-nitric acid method for 1 minute at a temperature of 90°C.

Next, the chemically polished specimen is anodized at a currentconcentration of 150 A/m² in a 15% sulfuric acid bath to form ananodized coating having a thickness of 10 μm. Then, the anodizedspecimen is immersed in boiling water to perform a sealing treatment onthe anodized coating. In this way, the specimen is subjected to asurface treatment (anodization).

Subsequently, the surface-treated (anodized) surface of the specimen isvisually observed. First, the specimen is observed from a viewpointvertical to a surface thereof, and the specimen having no surfacedefect, such as a scale-like pattern, a streak pattern, a mottledpattern, or a point defect, generated on its surface is determined to beacceptable. Further, the specimen is observed from a viewpoint at anangle of 30° with respect to its surface, and the specimen whose lightreflection state on its surface is uniform is determined to bedesirable.

Among the above-described surface defects, the scale-like pattern is apattern looking like scales along a grain boundary (a pattern in whichcrystal grains are seen more conspicuously) generated as a result ofetching the compounds precipitated on the grain boundary at pretreatmentof the surface treatment, in a case where the metallographic structureis an equigranular recrystallized structure. The streak pattern is apattern looking like a streak along a grain boundary generated as aresult of etching the compounds precipitated on the grain boundary atpretreatment of the surface treatment, in a case where themetallographic structure is a fibrous structure. The mottled pattern isa pattern generated because differences in the crystal grain size makethe crystal grains partially coarse or fine and such larger and smallercrystal grains look like mottles after the surface treatment. The pointdefect is caused when, for example, coarse compounds come off by beingetched. Concave pits are formed in a position where the compounds werepresent, and such concave pits look like points after the surfacetreatment.

TABLE 1 Chemical Composition (mass %) Mass Ratio Specimen Zn Mg Cu Zr SiFe Mn Cr Ti Al (Zn/Mg) 1 6.52 1.11 0.01 0.07 0.11 0.08 0.02 0.02 0.03bal. 5.87 2 8.47 0.98 0.04 0.06 0.08 0.15 0.01 0.03 0.01 bal. 8.64 37.05 1.11 0.08 0.07 0.18 0.23 0.03 0.02  0.008 bal. 6.35 4 7.99 1.210.01 0.06 0.14 0.21 0.02 0.03 0.03 bal. 6.60 5 7.13 0.52 0.02 0.08 0.230.09 0.02 0.03 0.02 bal. 13.71  6 8.11 1.48 0.05 0.07 0.15 0.22 0.020.02 0.01 bal. 5.48 7 7.88 1.05 0.02 0.08 0.12 0.20 0.01 0.03 0.03 bal.7.50 8 8.12 1.29 0.06 0.06 0.13 0.08 0.03 0.03 0.01 bal. 6.29 9 6.940.99 0.09 0.07 0.09 0.09 0.02 0.02 0.03 bal. 7.01 10 7.31 1.19 0.03 0.050.11 0.18 0.02 0.03  0.008 bal. 6.14 11 7.22 1.35 0.04 0.09 0.17 0.230.03 0.03 0.02 bal. 5.35 12 6.99 1.25 0.03 0.08 0.26 0.11 0.02 0.03 0.01bal. 5.59 13 7.34 0.82 0.07 0.06 0.13 0.25 0.02 0.03  0.009 bal. 8.95 146.98 0.94 0.02 0.06 0.22 0.09 0.04 0.02  0.009 bal. 7.43 15 6.81 1.110.07 0.08 0.18 0.18 0.03 0.04 0.03 bal. 6.14 16 7.99 1.31 0.07 0.07 0.210.20 0.01 0.02  0.001 bal. 6.10 17 8.10 0.98 0.01 0.06 0.16 0.08 0.020.01 0.04 bal. 8.27 18 7.29 1.44 0.05 0.08 0.14 0.23 0.03 0.02 0.02 bal.5.06 19 8.42 0.53 0.08 0.07 0.20 0.15 0.02 0.03 0.01 bal. 15.89  20 7.621.07 0.03 0.06 0.12 0.21 0.02 0.02  0.009 bal. 7.12 21 8.21 0.58 0.080.07 0.09 0.18 0.01 0.03 0.01 bal. 14.16  22 7.88 1.32 0.05 0.06 0.150.09 0.03 0.02 0.03 bal. 5.97 23 7.05 1.15 0.08 0.06 0.11 0.17 0.02 0.01 0.009 bal. 6.13 24 6.45 1.09 0.03 0.08 0.08 0.14 0.03 0.01 0.03 bal.5.92 25 8.56 1.23 0.06 0.08 0.23 0.18 0.01 0.02  0.008 bal. 6.96 26 6.770.47 0.07 0.06 0.09 0.16 0.02 0.01 0.02 bal. 14.40  27 7.87 1.54 0.050.07 0.17 0.18 0.01 0.02 0.01 bal. 5.11 28 8.11 1.02 0.11 0.08 0.18 0.130.03 0.03 0.03 bal. 7.95 29 7.88 1.11 0.02 0.04 0.21 0.08 0.03 0.02 0.03bal. 7.10 30 6.89 1.21 0.01 0.12 0.16 0.15 0.03 0.01 0.02 bal. 5.69 317.11 1.19 0.04 0.07 0.32 0.21 0.01 0.01 0.01 bal. 5.97 32 7.96 0.88 0.070.08 0.22 0.33 0.02 0.03 0.03 bal. 9.05 33 6.96 1.12 0.07 0.06 0.18 0.180.05 0.02 0.01 bal. 6.21 34 7.33 0.91 0.05 0.07 0.14 0.14 0.02 0.05 0.009 bal. 8.05 35 8.01 0.92 0.03 0.06 0.20 0.08 0.03 0.03  0.0008 bal.8.71 36 7.77 1.22 0.05 0.08 0.13 0.21 0.02 0.01 0.07 bal. 6.37 37 6.971.41 0.03 0.08 0.08 0.12 0.03 0.02 0.01 bal. 4.94 38 8.44 0.52 0.06 0.070.23 0.08 0.02 0.01 0.02 bal. 16.23 

TABLE 2 Resistance Metallographic Structure Observation Surface QualityMechanical Properties Toughness to SCC Average Grain Defect Tensil YieldElon- Impact Stress Metallo- grain diameter after Light strengthstrength gation Bending value corrosion graphic diameter differencesurface reflection Specimen (MPa) (MPa) (%) test (J/cm²) crackingstructure (μm) (μm) treatment state 1 344 319 19 ◯ 19.2 NoneEquigranular 356 269 None Uniform 2 383 355 16 ⊚ 17.6 None Equigranular401 245 None Uniform 3 388 362 16 ⊚ 17.1 None Equigranular 365 223 NoneUniform 4 403 378 15 ⊚ 16.9 None Equigranular 297 262 None Uniform 5 338311 20 ◯ 19.5 None Equigranular 321 278 None Uniform 6 412 381 14 ⊚ 16.9None Equigranular 332 265 None Uniform 7 396 367 16 ⊚ 17.0 NoneEquigranular 342 281 None Uniform 8 404 376 15 ⊚ 16.8 None Equigranular358 276 None Uniform 9 371 345 17 ◯ 17.9 None Equigranular 367 254 NoneUniform 10 379 354 17 ⊚ 17.5 None Equigranular 376 243 None Uniform 11386 359 16 ⊚ 17.3 None Equigranular 234 259 None Uniform 12 391 362 16 ⊚17.0 None Equigranular 298 228 None Uniform 13 370 339 18 ◯ 18.4 NoneEquigranular 432 252 None Uniform 14 353 324 19 ◯ 18.7 None Equigranular339 261 None Uniform 15 376 349 16 ◯ 18.1 None Equigranular 382 281 NoneUniform 16 407 379 15 ⊚ 16.7 None Equigranular 412 237 None Uniform 17377 348 17 ◯ 18.0 None Equigranular 399 231 None Uniform 18 397 365 16 ⊚16.6 None Equigranular 288 256 None Uniform 19 339 314 21 ◯ 22.8 NoneEquigranular 340 267 None Uniform 20 368 343 17 ◯ 17.3 None Equigranular383 283 None Uniform 21 338 310 22 ◯ 24.4 None Equigranular 299 255 NoneUniform 22 401 372 15 ⊚ 17.1 None Equigranular 305 271 None Uniform 23363 336 20 ◯ 18.6 None Equigranular 389 312 None Partially nonuniform 24303 272 24 X 20.9 None Equigranular 410 234 None Uniform 25 — — — — — —— — — — — 26 292 264 25 X 22.1 None Equigranular 399 242 None Uniform 27402 381 15 ⊚ 16.8 None Equigranular 421 269 Scale- Nonuniform likepatterns 28 377 352 17 ⊚ 17.8 None Equigranular 449 255 YellowishUniform 29 401 372 16 ⊚ 17.0 None Coarse and — — Mottled Nonuniformnonuniform patterns 30 379 353 17 ⊚ 17.9 None Fibrous — — StreakNonuniform patterns 31 365 339 18 ◯ 18.5 None Fibrous — — StreakNonuniform patterns 32 375 348 17 ◯ 18.2 None Fibrous — — StreakNonuniform patterns 33 386 357 16 ⊚ 17.7 None Fibrous — — StreakNonuniform patterns 34 371 343 18 ◯ 18.1 None Fibrous — — StreakNonuniform patterns 35 376 345 18 ◯ 18.4 None Equigranular — — MottledNonuniform patterns 36 379 351 17 ⊚ 17.6 None Equigranular 290 267 PointNonuniform defect 37 365 339 19 ◯ 14.1 None Equigranular 391 246 NoneUniform 38 336 311 21 ◯ 25.3 Cracking Equigranular 412 268 None Uniformgenerated

Evaluation results of the respective specimen are shown in Table 2. Asfor the specimens that were not determined to be acceptable (that weredetermined to be unacceptable), evaluation results or the like thereofare indicated with underlines applied thereto in Table 2.

As can be seen from Table 2, Specimens 1 to 23, whose metallographicstructures were equigranular recrystallized structures, were determinedto be acceptable or to be acceptable and also desirable in allevaluation items, that is, in terms of the mechanical properties (theyield strength and the bending test), the toughness (the impact value),the resistance to SCC (the stress corrosion cracking), themetallographic structure observation (the metallographic structure, theaverage grain diameter, and the grain diameter difference), and thesurface quality (the defect after surface treatment, and the lightreflection state). In sum, Specimens 1 to 23 exhibited excellentproperties in terms of the strength, the toughness, and the surfacequality, and also exhibited excellent properties in terms of theresistance to SCC.

As for Specimen 23, although no defect after surface treatment wasobserved, the light reflection state was partially nonuniform becausethe grain diameter difference among the crystal grains (the differencebetween the maximum value and the minimum value) was slightly large.However, such partial nonuniformity was not bad enough to be a problemin the surface quality. Specimen 23 was determined to be acceptable orto be acceptable and also desirable in all of the evaluation items otherthan the light reflection state. In sum, Specimen 23 exhibited excellentproperties in terms of the strength, the toughness, and the surfacequality, and also exhibited excellent properties in terms of theresistance to SCC.

Specimen 24, whose Zn content was too low, was determined to beunacceptable in terms of the yield strength because the strengthimproving effect was not sufficiently obtained. On the other hand,Specimen 25, whose Zn content was too high, was poor in the hotworkability, resulting in difficulty in performing hot extrusion withactually used facilities.

Specimen 26, whose Mg content was too low, was determined to beunacceptable in terms of the yield strength because the strengthimproving effect was not sufficiently obtained. On the other hand,Specimen 27, whose Mg content was too high, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause coarse compounds were present on the grain boundary to generatescale-like patterns on the anodized surface.

Specimen 28, whose Cu content was too high, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause its anodized surface was yellowish in color tone.

Specimen 29, whose Zr content was too low, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause a coarse and nonuniform recrystallized structure was formed togenerate mottled patterns on the anodized surface. On the other hand,Specimen 30, whose Zr content was too high, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause a fibrous structure was formed to generate streak patterns onthe anodized surface.

Specimen 31, whose Si content was too high, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause a fibrous structure was formed to generate streak patterns onthe anodized surface.

Specimen 32, whose Fe content was too high, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause a fibrous structure was formed to generate streak patterns onthe anodized surface.

Specimen 33, whose Mn content was too high, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause a fibrous structure was formed to generate streak patterns onthe anodized surface.

Specimen 34, whose Cr content was too high, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause a fibrous structure was formed to generate streak patterns onthe anodized surface.

Specimen 35, whose Ti content was too low, was determined to beunacceptable due to appearance of the defect after surface treatmentbecause the structure of the ingot was coarse and the metallographicstructure subjected to hot extrusion was nonuniform to generate mottledpatterns on the anodized surface. On the other hand, Specimen 36, whoseTi content was too high, was determined to be unacceptable due toappearance of the defect after surface treatment because coarseintermetallic compounds were generated to cause a point defect on theanodized surface.

Specimens 27 and 29 to 36, which were determined to be unacceptable interms of the defect after surface treatment, were nonuniform in thelight reflection state.

Specimen 37, whose mass ratio (Zn/Mg) was too low, was determined to beunacceptable in terms of the impact value (toughness) because the impactvalue was less than 15. On the other hand, Specimen 38, whose mass ratio(Zn/Mg) was too high, was determined to be unacceptable in terms of thestress corrosion cracking (resistance to SCC) because a stress corrosioncracking was generated in the test of resistance to SCC.

Example 2

Examples in the production method for the above-described aluminum alloymaterial will be described through comparison with comparative examples,with reference to Table 3 and Table 4. The below-described examples showone embodiment of the present disclosure, and the present disclosure isnot limited to these.

In this example, as shown in Table 3, a plurality of specimens(examples: Specimens A to H, comparative examples: Specimens I to N) ofthe aluminum alloy material were prepared under different productionconditions, and various evaluations were conducted on each specimen. Thechemical components of the aluminum alloy material were similar to thoseof Specimen 10 or Specimen 11 (see Table 1) of Example 1 describedabove. A preparation method for the specimens will be described below.Various evaluation methods were similar to those in the above-describedExample 1.

<Method for Preparing Specimen>

A cylindrical ingot (billet) having a diameter of 90 mm containingchemical components similar to those of Specimen 10 or Specimen 11 (seeTable 1) of the above-described Example 1 is forged by semicontinuouscasting. Then, a homogenizing treatment is performed in which the ingotis heated at a temperature and for a period of time shown in Table 3.Subsequently, the ingot is subjected to hot extrusion with thetemperature of the ingot maintained at 520° C. In this way, an extrudedmaterial having a width of 150 mm and a thickness of 10 mm is obtained.

Next, a quenching treatment is performed in which the extruded materialsubjected to hot extrusion is cooled to 100° C. at a cooling rate of1500° C./min. Then, the quenched extruded material is cooled to roomtemperature, and an artificial aging treatment is performed in which theextruded material is heated at a temperature of 140° C. for 12 hours. Inthis way, the specimen of the aluminum alloy material (extrudedmaterial) is obtained.

TABLE 3 Homogenizing treatment Alloy Temperature Time Specimen (SpecimenNo.) (° C.) (h) A 10 542 10  B 11 C 10 576 8 D 11 E 10 559 1 F 11 G 10565 24  H 11 I 10 535 10  J 11 K 10 584 8 L 11 M 10 560   0.5 N 11

TABLE 4 Resistance Metallographic Structure Observation Surface QualityMechanical Properties Toughness to SCC Average Grain Defect Tensil YieldElon- Impact Stress Metallo- grain diameter after Light strengthstrength gation Bending value corrosion graphic diameter differencesurface reflection Specimen (MPa) (MPa) (%) test (J/cm²) crackingstructure (μm) (μm) treatment state A 381 356 17 ⊚ 17.5 NoneEquigranular 379 244 None Uniform B 389 362 16 ⊚ 17.3 None Equigranular343 261 None Uniform C 379 355 16 ⊚ 17.4 None Equigranular 401 233 NoneUniform D 385 360 17 ⊚ 17.2 None Equigranular 299 259 None Uniform E 382356 17 ⊚ 17.6 None Equigranular 334 229 None Uniform F 388 361 17 ⊚ 17.1None Equigranular 420 282 None Uniform G 381 353 16 ⊚ 17.3 NoneEquigranular 405 245 None Uniform H 387 364 16 ⊚ 17.5 None Equigranular329 229 None Uniform I 380 353 17 ⊚ 17.7 None Fibrous — — StreakNonuniform patterns J 385 358 16 ⊚ 17.6 None Fibrous — — StreakNonuniform patterns K — — — — — — — — — — — L — — — — — — — — — — — M379 354 17 ⊚ 17.4 None Coarse and — — Mottled Nonuniform nonuniformpatterns N 386 359 16 ⊚ 17.2 None Coarse and — — Mottled Nonuniformnonuniform patterns

As can be seen from Table 4, Specimens A to H, whose metallographicstructures were equigranular recrystallized structures, were determinedto be acceptable or to be desirable in all evaluation items, that is, interms of the mechanical properties (the yield strength and the bendingtest), the toughness (the impact value), the resistance to SCC (thestress corrosion cracking), the metallographic structure observation(the metallographic structure, the average grain diameter, and the graindiameter difference), and the surface quality (the defect after surfacetreatment, and the light reflection state). In sum, Specimens A to Hexhibited excellent properties in terms of the strength, the toughness,and the surface quality, and also exhibited excellent properties interms of the resistance to SCC.

In Specimens I and J, which were each homogenized at too low atemperature, Al—Zr-based compounds having an L1₂ structure were presentand fibrous structures were formed. Thus, Specimens I and J weredetermined to be unacceptable due to appearance of the defect aftersurface treatment because streak patterns were generated on the anodizedsurface.

In Specimens K and L, which were each homogenized at too high atemperature, local melting occurred to make it difficult to perform hotextrusion in the actually used facilities.

Specimens M and N, which were each homogenized for too short a time,were determined to be unacceptable due to appearance of the defect aftersurface treatment because their metallographic structures after hotextrusion were nonuniform to generate mottled patterns on the anodizedsurface.

In the above-described Examples 1 and 2, the extruded materials wereevaluated as one embodiment of the aluminum alloy material of thepresent disclosure. However, results similar to those of theabove-described Examples 1 and 2 are obtained also in a case of otherembodiments, such as plate materials, for example.

1. An aluminum alloy material having a chemical composition comprising:Zn: more than 6.5% (mass %, same applies hereafter) and 8.5% or less;Mg: 0.5% or more and 1.5% or less; Cu: 0.10% or less; Fe: 0.30% or less;Si: 0.30% or less; Mn: less than 0.05%; Cr: less than 0.05%; Zr: 0.05%or more and 0.10% or less; and Ti: 0.001% or more and 0.05% or less, abalance comprising Al and inevitable impurities, wherein a mass ratio ofZn to Mg (Zn/Mg) is 5 or more and 16 or less, and wherein ametallographic structure comprises a recrystallized structure, which isequigranular.
 2. The aluminum alloy material according to claim 1,wherein the recrystallized structure comprises crystal grains having anaverage grain diameter of 500 μm or less in a cross-section parallel toa direction orthogonal to a working direction, and a difference betweena maximum value and a minimum value of grain diameters of the crystalgrains is less than 300 μm.
 3. A production method for an aluminum alloymaterial, a metallographic structure of which comprises an equigranularrecrystallized structure, the method comprising: preparing an ingothaving a chemical composition comprising: Zn: more than 6.5% (mass %,same applies hereafter) and 8.5% or less; Mg: 0.5% or more and 1.5% orless; Cu: 0.10% or less; Fe: 0.30% or less; Si: 0.30% or less; Mn: lessthan 0.05%; Cr: less than 0.05%; Zr: 0.05% or more and 0.10% or less;and Ti: 0.001% or more and 0.05% or less, a balance comprising Al andinevitable impurities, wherein a mass ratio of Zn to Mg (Zn/Mg) is 5 ormore and 16 or less; and performing a homogenizing treatment in whichthe ingot is heated at a temperature higher than 540° C. and 580° C. orlower for 1 hour or longer and 24 hours or shorter.